There are a number of prior art graphics systems which incorporate the capability of combining two sources of video into a composite image. Representative of such prior art is U.S. Pat. No. 4,498,098, Stell, that describes an apparatus for combining a video signal with graphics and text from a computer. This particular patent shows the combination of two video signals by having both sources of video in an RGB format (that is the video signal is converted into its component red, green and blue signals) with a multiplexer switch selecting which of the two sources is to be displayed for each pixel of the display. Such a technique is unlike the present invention wherein a video source is converted into digital chrominance and luminance data samples which are stored in a frame buffer, along with a generated chrominance reference synchronization signal. This signal is later read with the chrominance and luminance data samples to form an RGB formatted output. Since such reading is independent of the data writing operation, the read data can be combined in any desired manner with graphic data so as to generate a desired overall effect.
Although U.S. Pat. No. 4,654,708, de la Guardia, et al, is directed to a digital video synchronization circuit, the technique disclosed in this reference converts an incoming synchronization signal to a digital format which is then transferred to a microprocessor which is programmed to recognize a particular synchronization pattern. The present invention uses a digital television integrated circuit chip set and stores chrominance reference synchronization information within the frame buffer so as to insure chrominance synchronization of the read chrominance and luminance data samples regardless of when such data is read from the frame buffer.